Remove gnutls from the travis build

Causing spurious fails of ssl-opt.sh. Likely a version issue. It would be
better to investigate the exact problem, and maybe adapt ssl-opt.sh to be
check for a minimum version just as compat.sh does, but this is a quick fix.
diff --git a/.travis.yml b/.travis.yml
index 7f100c3..0a51e7e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
 - clang
 - gcc
 before_install: sudo apt-get update
-install: sudo apt-get install gnutls-bin valgrind perl
+install: sudo apt-get install valgrind perl
 script:
 - cmake -D CMAKE_BUILD_TYPE:String="Check" .
 - make