Reducing travis-ci build matrix

Builds were taking > 10 mins, which travis-ci recommends as a limit you
don't go over.
Fixes to alternate SSL backends to not crash
Disable alternate SSL backends

Not enough time/effort has been put into making the cyaSSL, GnuTLS, and
PolarSSL backends robust, so they've been disabled. They can be compiled
by adding -DAMQP_USE_UNTESTED_SSL_BACKEND to the compiler flags