remove 2.7 testing, update before_install

- since bundler 2 is not compatible prior to 2.7 and I don't want to
figure out how to switch based on ruby versions, just skip testing 2.7
for a while. I'll update this when the CF team that uses this gem asks
me to.

.travis.yml

@@ -4,5 +4,4 @@ rvm:
   - 2.5.5
   - 2.5
   - 2.6
-  - 2.7
-before_install: gem install bundler -v 1.16.0
+before_install: gem install bundler -v 1.17.0