Don't run travis tests on master.

This saves us builders for more helpful purposes.

.travis.yml

@@ -152,16 +152,30 @@ before_script:
       echo "#### Disk usage before running script:";
       df -h;
       du . | sort -nr | head -n100
+  # If we are building a pull request, do the build if $ALLOW_PR == 1
+  # Otherwise, do the build if we are on the auto branch, or the try branch and $ALLOW_TRY == 1
   - >
-      if [[ "$ALLOW_PR" = "1" || "$TRAVIS_BRANCH" == "auto" || ( "$ALLOW_TRY" = "1" && "$TRAVIS_BRANCH" = "try" ) ]]; then
+      if [[ "$TRAVIS_PULL_REQUEST" != "false" ]]; then
+          if [[ "$ALLOW_PR" == "1" ]]; then
+              SKIP_BUILD=false;
+          else
+              SKIP_BUILD=true;
+          fi
+      elif [[ "$TRAVIS_BRANCH" == "auto" || ( "$ALLOW_TRY" == "1" && "$TRAVIS_BRANCH" == "try" ) ]]; then
+          SKIP_BUILD=false;
+      else
+          SKIP_BUILD=true;
+      fi
+
+      if [[ "$SKIP_BUILD" == true ]]; then
+          export RUN_SCRIPT="echo 'skipping, not a full build'";
+      else
           RUN_SCRIPT="stamp src/ci/init_repo.sh . $HOME/rustsrc";
           if [ "$TRAVIS_OS_NAME" = "osx" ]; then
               export RUN_SCRIPT="$RUN_SCRIPT && stamp src/ci/run.sh";
           else
               export RUN_SCRIPT="$RUN_SCRIPT && stamp src/ci/docker/run.sh $IMAGE";
           fi
-      else
-          export RUN_SCRIPT="echo 'skipping, not a full build'";
       fi
 
 script: