Auto merge of #21594 - gterzian:try_to_fix_travis, r=jdm

Try to fix travis

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #21592 (github issue number if applicable).

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.travis.yml

@@ -43,6 +43,8 @@ matrix:
       before_cache:
         - ./mach clean-nightlies --keep 2 --force
         - ./mach clean-cargo-cache --keep 2 --force
+        - rm -rf $HOME/.cargo/registry
+        - rm -rf $HOME/.servo/target
       env:
         CC=gcc-6
         CXX=g++-6