Skip explicit run of cargo build on CI

Running `cargo test` produces a set of normal build artifacts in
addition to test versions of each.  (That is, running `cargo build`
after `cargo test` is a no-op.)  By removing the explicit `cargo build`
invocation the test dependencies can begin building in parallel a bit
sooner.

Author: jtgeibel

.travis.yml

@@ -60,16 +60,13 @@ matrix:
       script:
         - cargo fmt -- --check
         - cargo clippy --all-targets --all-features --all
-        - cargo build
         - cargo test
         - npm test
     - rust: beta
       script:
-        - cargo build
         - cargo test
     - rust: nightly
       script:
-        - cargo build
         - cargo test
 
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