Improved testing on Travis
- .travis.sh: Rewrite for clarity and speed. Testing shows that valgrind and ASan don't play well together. So split their executions.
- .travis.yml: Enable ccache for builds. Also try to update libc6 if possible
- .travis.yml: Do not run on OSX with CC=gcc. This is because on OSX, gcc is only a wrapper around clang.
- .travis_setup.sh: Cosmetic changes only
- Makefile.am (check-coverage): Explcicitly disable valgrind tests when checking coverage
- tests/libtest.c (wget_test): Use a provided suppressions file for valgrind. This is required since the valgrind version on Travis seems to be out of sync with libc. This causes valgrind to report a memory bug falsely.
- tests/valgrind-suppressions: New file with some valgrind suppressions. Included suppressions are for a valgrind/libc bug and to suppress reachable memory messages from within gnutls