travis: test PRs to master but not commits

Author: japaric

.travis.yml

@@ -4,12 +4,15 @@ matrix:
   include:
     - env: TARGET=x86_64-unknown-linux-gnu
       rust: nightly
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
 
     - env: TARGET=thumbv6m-none-eabi
       rust: nightly
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
 
     - env: TARGET=thumbv7m-none-eabi
       rust: nightly
+      if: (branch = staging OR branch = trying) OR (type = pull_request AND branch = master)
 
 before_install: set -e
 
@@ -29,6 +32,7 @@ before_cache:
 
 branches:
   only:
+    - master
     - staging
     - trying