Use "travis_retry" to install gRPC.

Retrying the gRPC installation is necessary because it downloads several
external dependencies and the download of these may fail.

.travis.yml

@@ -25,7 +25,7 @@ before_install:
   - ps -A -o pid,rss,vsz,cmd --sort -rss
   - ps -A -o pid,rss,vsz,cmd --sort -rss | awk '{ sum += $2 } END { print sum / 1024.0, "MB used" }'
   - ps -A -o pid,rss,vsz,cmd --sort -rss | awk '{ sum += $3 } END { print sum / 1024.0, "MB virtual memory used" }'
-  - sudo bash -v travis/install_grpc.sh
+  - travis_retry sudo bash -v travis/install_grpc.sh
   - free -m
   - ps -A -o pid,rss,vsz,cmd --sort -rss
   - ps -A -o pid,rss,vsz,cmd --sort -rss | awk '{ sum += $2 } END { print sum / 1024.0, "MB used" }'