Ensure all compile jobs declare their compiler type

Some of the travis compile jobs were not declaring the 'compiler'
property. While this doesn't currently cause us any problems,
we should be consistent between the jobs. It also slightly improves
the chance of jobs for a new PR to pickup a useful seed cache, since
Travis looks for caches in the following order:
   1 - If the PR has previously been built, the cache from that build
   2 - else, the cache for the target branch (e.g. develop)
   3 - else, a cache for the compiler type (e.g. gcc)

Author: chrisr-diffblue

.travis.yml

@@ -169,6 +169,7 @@ jobs:
     # cmake build using g++-5
     - stage: Test different OS/CXX/Flags
       os: linux
+      compiler: gcc
       cache: ccache
       env:
         - BUILD_SYSTEM=cmake
@@ -189,6 +190,7 @@ jobs:
 
     - stage: Test different OS/CXX/Flags
       os: osx
+      compiler: clang
       cache: ccache
       before_install:
         - HOMEBREW_NO_AUTO_UPDATE=1 brew install ccache