Travis: make cache dependent on emscripten version

.travis.yml

@@ -122,6 +122,16 @@ matrix:
               - SOLC_TESTS=Off
               - ZIP_SUFFIX=emscripten
               - SOLC_STOREBYTECODE=On
+              # Travis doesn't seem to support "dynamic" cache keys where we could include
+              # the hashes of certain files. Our CircleCI configuration contains the hash of
+              # relevant emscripten files.
+              #
+              # It is important to invalidate the cache with each emscripten update, because
+              # dependencies, such as boost, might be broken otherwise.
+              #
+              # This key here has no significant on anything, apart from caching. Please keep
+              # it in sync with the version above.
+              - EMSCRIPTEN_VERSION_KEY="1.38.22"
 
         # OS X Mavericks (10.9)
         # https://en.wikipedia.org/wiki/OS_X_Mavericks