Use explicit gemfile in bundle exec in CI build

These calls to `bundle exec` all come after a call to `bundle install
--gemfile=xxx` and the latter will only take effect if we use `bundle
exec --gemfile=xxx` (or if the `BUNDLE_GEMFILE` is set).

In practice all these cases are using the default `Gemfile`, so these
changes don't actually have any effect. However, I think making things
explicit makes the code easier to follow.

.circleci/config.yml

@@ -27,7 +27,7 @@ jobs:
           condition:
             equal: [ "Gemfile", << parameters.gemfile >>]
           steps:
-            - run: bundle exec rake test
+            - run: bundle exec --gemfile=<< parameters.gemfile >> rake test
       - when:
           condition:
             matches:
@@ -48,7 +48,7 @@ jobs:
               - equal: [ "ruby:latest", << parameters.docker-image >> ]
               - equal: [ "Gemfile", << parameters.gemfile >>]
           steps:
-            - run: bundle exec rake test:performance
+            - run: bundle exec --gemfile=<< parameters.gemfile >> rake test:performance
             - run: MOCHA_GENERATE_DOCS=1 bundle install --gemfile=<< parameters.gemfile >>
             - run: MOCHA_GENERATE_DOCS=1 rake yardoc
   lint:
@@ -60,7 +60,7 @@ jobs:
       - run: gem --version
       - run: bundle --version
       - run: bundle install --gemfile=Gemfile
-      - run: bundle exec rake lint
+      - run: bundle exec --gemfile=Gemfile rake lint
 
 workflows:
   build-all: