Use travis_retry for installing the modules

https://irclog.perlgeek.de/perl6/2017-01-06#i_13868005

Author: samcv

.travis.yml

@@ -30,9 +30,9 @@ before_install:
 install:
   - rakudobrew build-zef
   - export PATH="$PATH:/home/travis/.rakudobrew/moar-nom/install/share/perl6/site/bin"
-  - zef --/tap-harness --force --/test install LWP::Simple
-  - zef --/tap-harness --depsonly install .
-  - zef --/tap-harness install Pod::To::HTML
+  - travis_retry zef --/tap-harness --force --/test install LWP::Simple
+  - travis_retry zef --/tap-harness --depsonly install .
+  - travis_retry zef --/tap-harness install Pod::To::HTML
 
 script:
   - make test