Travis: Skip gcc on Linux with default settings

It's redundant, because all the non-default builds use gcc on Linux.

Part of 27252.

.travis.yml

@@ -62,6 +62,11 @@ matrix:
     ## gcc on OSX is less useful, because the default compiler is clang.
     - compiler: gcc
       os: osx
+    ## gcc on Linux with no env is redundant, because all the custom builds use
+    ## gcc on Linux
+    - compiler: gcc
+      os: linux
+      env:
 
 ## We don't need sudo. (The "apt:" stanza after this allows us to not need
 ## sudo; otherwise, we would need it for getting dependencies.)

changes/ticket27252

   o Minor features (continuous integration):
     - Skip gcc on OSX in Travis CI, it's rarely used.
       Skip a duplicate hardening-off build in Travis on Tor 0.2.9.
+      Skip gcc on Linux with default settings, because all the non-default
+      builds use gcc on Linux.
       Implements ticket 27252.