ci: Remove Travis CI caching workaround

Travis CI now takes into account the environment variables when
generating the cache hash. So the workaround of appending random unique
stuff to "compiler" is now unnecessary.

.travis.yml

@@ -8,19 +8,19 @@ cache: ccache
 matrix:
   include:
     - env: VERSION=6 BITS=32
-      compiler: gcc-6-32
+      compiler: gcc
       os: linux
     - env: VERSION=5 BITS=32
-      compiler: gcc-5-32
+      compiler: gcc
       os: linux
     - env: VERSION=4.9 BITS=32
-      compiler: gcc-4.9-32
+      compiler: gcc
       os: linux
     - env: VERSION=3.8 BITS=32
-      compiler: clang-3.8-32
+      compiler: clang
       os: linux
     - env: VERSION=4.9 BITS=64
-      compiler: gcc-4.9-64
+      compiler: gcc
       os: linux
 
 before_install: