MAINT: Disable cargo incremental for all travis configurations

It slows down the build a bit, and we have no benefit from it in
travis.

Author: bluss

.travis.yml

@@ -10,16 +10,17 @@ matrix:
     - rust: stable
       env:
        - FEATURES='test docs'
-       - CARGO_INCREMENTAL=0
     - rust: beta
       env:
        - FEATURES='test docs'
-       - CARGO_INCREMENTAL=0
     - rust: nightly
       env:
        - FEATURES='test docs'
        - IS_NIGHTLY=1
-       - CARGO_INCREMENTAL=0
+env:
+  global:
+    - HOST=x86_64-unknown-linux-gnu
+    - CARGO_INCREMENTAL=0
 branches:
   only:
     - master