Update port compiler version for rebar3

Update the port compiler version from 1.11.0 to 1.15.0 to prevent
rebar3 build errors.

Fixes #495

Author: vinoski

rebar.config

@@ -22,7 +22,7 @@
              {clean, "./scripts/rebar-pre-script clean"}]}.
 
 {plugins, [
-   {pc, ".*", {git, "https://github.com/blt/port_compiler.git", {tag, "v1.11.0"}}}
+   {pc, ".*", {git, "https://github.com/blt/port_compiler.git", {tag, "v1.15.0"}}}
 ]}.
 
 {provider_hooks, [