Excludes external libraries from code coverage

They provide far more code than we actually use.
And we certainly don't want to test all that code!

.travis.yml

@@ -25,7 +25,7 @@ install:
 
 after_success:
   # exclude some directories from profiling (.libs is from autotools)
-  - export EXCLUDE_COVERAGE="--exclude sassc --exclude sass-spec --exclude .libs"
+  - export EXCLUDE_COVERAGE="--exclude sassc --exclude sass-spec --exclude .libs --exclude json.cpp --exclude cencode.c --exclude .libs --exclude b64 --exclude utf8.h --exclude utf8_string.cpp --exclude test"
   # debug via gcovr
   - gcovr -r .
   # debug via coveralls (dump result for futher analyzing)