Don't fail if sccache is present in CI cache

See
- https://travis-ci.org/scipr-lab/zexe/jobs/642453454?utm_medium=notification&utm_source=github_status
- https://travis-ci.org/scipr-lab/zexe/jobs/642453582?utm_medium=notification&utm_source=github_status

Read for details:
rust-lang/cargo#6727

.travis.yml

@@ -5,7 +5,7 @@ before_install:
   - export SCCACHE_DIR="$TRAVIS_HOME/.cache/sccache"
 
 install:
-  - cargo install sccache
+  - (cargo install sccache || true)
   - export PATH="$PATH:$TRAVIS_HOME/.cargo/bin"
   - export RUSTC_WRAPPER="sccache"