Disable incremental compilation on CI

For some reason, incremental files are changing on CI even when no code
changes.  Turning off incremental compilation should help avoid
uploading a new cache with every single build.

This commit also removes `target/.rustc_info.json` from the CI cache

Author: jtgeibel

.travis.yml

@@ -14,6 +14,7 @@ env:
     - DATABASE_URL=postgres://postgres:@localhost/cargo_registry_test
     - TEST_DATABASE_URL=postgres://postgres:@localhost/cargo_registry_test
     - CARGO_TARGET_DIR=target
+    - CARGO_INCREMENTAL=0
     - PERCY_PARALLEL_TOTAL=2
     # Percy secrets are included here to enable Percy's GitHub integration
     # on community-submitted PRs

script/ci/prune-cache.sh

@@ -25,5 +25,7 @@ for name in $bin_names; do
     rm -v target/debug/deps/$normalized-*
 done
 
+rm -v target/.rustc_info.json
+
 echo "Final cache size:"
 du -hs target/debug