Add x86_64-unknown-linux-gnu as another regular job

Author: Susurrus

.travis.yml

@@ -15,14 +15,12 @@ env:
     - CRATE_NAME=nix
     - CARGO_TEST_THREADS=1
 
-    # default job
-    - TARGET=x86_64-unknown-linux-gnu
-
 matrix:
   # These are all the build jobs. Adjust as necessary. Comment out what you
   # don't need
   include:
     # Linux
+    - env: TARGET=x86_64-unknown-linux-gnu
     - env: TARGET=i686-unknown-linux-gnu
     - env: TARGET=i686-unknown-linux-musl
     # - env: TARGET=x86_64-unknown-linux-gnu  # this is the default job