travis: Only use GCC for crypto plugin tests

They are already build-tested with Clang via "all" and others.

.travis.yml

@@ -78,22 +78,11 @@ matrix:
     - env: TEST=printf-builtin LEAK_DETECTIVE=yes
     - env: TEST=printf-builtin LEAK_DETECTIVE=yes
       compiler: clang
+    # the crypto plugins are build-tested with clang via "all" above
     - env: TEST=botan
-    - env: TEST=botan
-      compiler: clang
     - env: TEST=botan LEAK_DETECTIVE=yes
-    - env: TEST=botan LEAK_DETECTIVE=yes
-      compiler: clang
     - env: TEST=openssl
-    - env: TEST=openssl
-      compiler: clang
     - env: TEST=openssl LEAK_DETECTIVE=yes
-    - env: TEST=openssl LEAK_DETECTIVE=yes
-      compiler: clang
     - env: TEST=gcrypt
-    - env: TEST=gcrypt
-      compiler: clang
     - env: TEST=gcrypt LEAK_DETECTIVE=yes
-    - env: TEST=gcrypt LEAK_DETECTIVE=yes
-      compiler: clang
     - env: TEST=apidoc