Freeze fpm to version v1.8.1 for travis CI builds

Use fpm version v1.8.1 on travis to handle issue
where symlinks were not copied to the packing staging
directory when using source=dest/ syntax

Can be removed when the bugfix in
jordansissel/fpm#1399 has been
merged and released

.travis.yml

@@ -40,7 +40,7 @@ env:
 #    packages:
 
 before_install:
-  - gem install fpm
+  - gem install fpm -v 1.8.1
   - if [[ -n "$TRAVIS_TAG" ]]; then
         curl -XGET -L -k 'https://api.bintray.com/content/jfrog/jfrog-cli-go/$latest/jfrog-cli-linux-amd64/jfrog?bt_package=jfrog-cli-linux-amd64' > /tmp/jfrog ;
         chmod a+x /tmp/jfrog ;