Prevent jobs from being terminated on Travis (#167)

* Prevent jobs from being terminated

* Fix installing clippy

* Fix command

* Run Clippy on nightly

.travis.yml

@@ -3,8 +3,13 @@ sudo: false
 dist: trusty
 
 cache:
-  cargo: true
   apt: true
+  directories:
+    - $HOME/.cargo
+    - $HOME/.rustup
+
+before_cache:
+  - rm -rf $HOME/.cargo/registry
 
 matrix:
   include:
@@ -28,9 +33,13 @@ before_install:
 before_script:
   - |
     if [[ "$TRAVIS_RUST_VERSION" == "nightly" ]]; then
-        ( ( cargo install clippy && export CLIPPY=true ) || export CLIPPY=false );
+      rustup component add clippy --toolchain=nightly
     fi
   - export PATH=$PATH:~/.cargo/bin
 
 script:
   - cargo check --all
+  - |
+    if [[ "$TRAVIS_RUST_VERSION" == "nightly" ]]; then
+      cargo clippy
+    fi