travis.yml: Actually trigger coveralls

Previously it was just erroring:
[[: command not found

This has been broken since #3638
diff --git a/.travis.yml b/.travis.yml
index d29b5ef..04cbe92 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -33,7 +33,7 @@
 
 after_success:
     # Upload to coveralls once, instead of for each job in the matrix
-  - if \[\[ "$TRAVIS_JOB_NUMBER" == *.1 \]\]; then ./gradlew :grpc-all:coveralls; fi
+  - if [[ "$TRAVIS_JOB_NUMBER" == *.1 ]]; then ./gradlew :grpc-all:coveralls; fi
   - bash <(curl -s https://codecov.io/bash)
 
 os: