Update travis to use clang 6

Signed-off-by: Ajmal Sharif <asharif10@bloomberg.net>

Author: HaltCatchFire

.travis.Dockerfile

@@ -2,18 +2,18 @@ FROM ubuntu:18.04
 
 # Install build dependencies
 RUN apt-get update && apt-get install -y \
-    clang-5.0 \
-    clang-format-5.0 \
-    clang-tools-5.0 \
-    clang-tidy-5.0 \
+    clang-6.0 \
+    clang-format-6.0 \
+    clang-tools-6.0 \
+    clang-tidy-6.0 \
     cmake \
-    libclang-5.0-dev \
+    libclang-6.0-dev \
     libfile-spec-native-perl \
     libgtest-dev
 
 # Set up clang compilers
-ENV CC=/usr/lib/llvm-5.0/bin/clang \
-    CXX=/usr/lib/llvm-5.0/bin/clang++
+ENV CC=/usr/lib/llvm-6.0/bin/clang \
+    CXX=/usr/lib/llvm-6.0/bin/clang++
 
 # Fix issues with gtest installation from ubuntu debian
 RUN cd /usr/src/gtest && \
@@ -23,17 +23,17 @@ RUN cd /usr/src/gtest && \
 
 # Fix issues with clang installation from ubuntu debian
 RUN mkdir -p /usr/lib/cmake && \
-    ln -s /usr/share/llvm-5.0/cmake /usr/lib/cmake/clang && \
-    for hdr in /usr/lib/llvm-5.0/include/clang/*; do \
+    ln -s /usr/share/llvm-6.0/cmake /usr/lib/cmake/clang && \
+    for hdr in /usr/lib/llvm-6.0/include/clang/*; do \
         ln -s $hdr /usr/include/clang/$(basename $hdr); \
     done && \
-    ln -s /usr/lib/llvm-5.0/include/clang-c /usr/include/clang-c && \
-    ln -s /usr/lib/llvm-5.0/include/llvm /usr/include/llvm && \
-    ln -s /usr/lib/llvm-5.0/include/llvm-c /usr/include/llvm-c && \
-    for lib in /usr/lib/llvm-5.0/lib/*; do \
+    ln -s /usr/lib/llvm-6.0/include/clang-c /usr/include/clang-c && \
+    ln -s /usr/lib/llvm-6.0/include/llvm /usr/include/llvm && \
+    ln -s /usr/lib/llvm-6.0/include/llvm-c /usr/include/llvm-c && \
+    for lib in /usr/lib/llvm-6.0/lib/*; do \
         ln -s $lib /usr/lib/$(basename $lib); \
     done && \
-    for bin in /usr/bin/*-5.0; do \
+    for bin in /usr/bin/*-6.0; do \
         ln -s $bin /usr/bin/$(basename $bin | rev | cut -d '-' -f2- | rev); \
     done
 
@@ -42,18 +42,18 @@ WORKDIR clangmetatool
 
 # Build tool, run tests, and do a test install
 RUN mkdir build && cd build && \
-    cmake -DClang_DIR=/usr/share/llvm-5.0/cmake .. && \
+    cmake -DClang_DIR=/usr/share/llvm-6.0/cmake .. && \
     make all test && \
     make install && \
     cd .. && rm -rf build
 
 # Fix includes for clangmetatool (due to ubuntu debian's clang)
-RUN ln -s /usr/lib/llvm-5.0/include/clangmetatool /usr/include/clangmetatool
+RUN ln -s /usr/lib/llvm-6.0/include/clangmetatool /usr/include/clangmetatool
 
 # Build skeleton
 RUN mkdir skeleton/build && cd skeleton/build && \
-    cmake -DClang_DIR=/usr/share/llvm-5.0/cmake \
-          -Dclangmetatool_DIR=/usr/share/llvm-5.0/cmake .. && \
+    cmake -DClang_DIR=/usr/share/llvm-6.0/cmake \
+          -Dclangmetatool_DIR=/usr/share/llvm-6.0/cmake .. && \
     make all && \
     make install && \
     cd - && rm -rf skeleton/build