Build environment needs to enforce Java version 1.6

[chreex]

hu.qgears.rtemplate.ast.AbstractAST$OffsetComparator calls Integer.compare(int x, int y), which is present from Java 1.7, whereas Bundle-RequiredExecutionEnvironment: JavaSE-1.6 is set in the MANIFEST.MF. As a consequence, Eclipse complains that the method is undefined if a real Java 6 execution environment is assigned to the JavaSE-1.6 execution environment. (Otherwise, if Java 7 or Java 8 JDK is assigned to JavaSE-1.6, it does not.)

[pulinyul]

For now, I have bumped the execution environment version of hu.qgears.rtemplate to Java 7 so the new version can be built everywhere.
For a proper solution, we not only need to enforce source and target version on the compiler (see: https://eclipse.org/tycho/sitedocs/tycho-compiler-plugin/compile-mojo.html#source and https://eclipse.org/tycho/sitedocs/tycho-compiler-plugin/compile-mojo.html#target) but also have to have all the required versions of JDKs installed and configured using: https://eclipse.org/tycho/sitedocs/tycho-compiler-plugin/compile-mojo.html#useJDK