refactor(travis): Remove Debug build configurations

They're not really any different from the release builds and only
elongate the build process. Also now that we have a minimal build, we
would have more than five build configurations which would make travis
limit itself to five concurrent builds, letting the last build wait on
the others.

Also it's just unnecessary load on the travis servers

.travis.yml

@@ -49,10 +49,6 @@ matrix:
       env: BUILD_TYPE=Release
       addons: {apt: {sources: *sources, packages: [*clang, *base_deps, *optional_deps]}}
 
-    - compiler: clang
-      env: BUILD_TYPE=Debug
-      addons: {apt: {sources: *sources, packages: [*clang, *base_deps, *optional_deps]}}
-
     - compiler: gcc
       env: BUILD_TYPE=Debug POLYBAR_BUILD_TYPE=tests BUILD_TESTS=ON
       addons: {apt: {sources: *sources, packages: [*gcc, *base_deps, *optional_deps]}}
@@ -65,10 +61,6 @@ matrix:
       env: BUILD_TYPE=Release
       addons: {apt: {sources: *sources, packages: [*gcc, *base_deps, *optional_deps]}}
 
-    - compiler: gcc
-      env: BUILD_TYPE=Debug
-      addons: {apt: {sources: *sources, packages: [*gcc, *base_deps, *optional_deps]}}
-
       # Minimal build, contains no optional dependencies. This makes sure that
       # we properly remove files from compilation that depend on libraries that
       # are not installed