Reduce number of storage types that are tested for performance reasons.

Compiler performance issues described in #52 are causing the `uom` build
to take a significant amount of time when all underlying storage types
are enabled. As additional quantities are added this performance gets
worse. This commit only tests a few types in order to keep the build
time within Appveyor and TravisCI limits.

.appveyor.yml

@@ -54,8 +54,8 @@ install:
 ## Build Script ##
 build: false
 test_script:
-  - cargo build --verbose --all-features
-  - cargo test --all --verbose --features serde
+  - cargo build --verbose --no-default-features --features "usize isize bigint rational bigrational f32 f64 si std use_serde"
+  - cargo test --all --verbose --features "use_serde"
 
 notifications:
     - provider: Email

.travis.yml

@@ -30,8 +30,8 @@ before_script: |
   (test $(grep "cargo-travis" /home/travis/.cargo/.crates.toml | sed -r "s/\"cargo-travis ([^\ ]+).+/\1/") = $(cargo search cargo-travis --limit 1 | sed -r "s/cargo-travis \(([^\)]+)\).+/\1/") || cargoo install cargo-travis --force)
 
 script: |
-  cargo build --verbose --all-features &&
-  (test "$TRAVIS_RUST_VERSION" == "1.20.0" || cargo test --all --verbose --features serde)
+  cargo build --verbose --no-default-features --features "usize isize bigint rational bigrational f32 f64 si std use_serde" &&
+  (test "$TRAVIS_RUST_VERSION" == "1.20.0" || cargo test --all --verbose --features "use_serde")
 
 after_success: |
   test "$TRAVIS_RUST_VERSION" != "stable" || cargo coveralls