Update Tarpaulin to v0.10.0

Also download a prebuilt binary from GitHub to speed up Travis builds.

The later 0.9.x and 0.10.0 release binaries seems to have been built
against a version of libssl that isn't available on Travis.

.travis.yml

@@ -12,11 +12,8 @@ cache: cargo
 install:
   - rustup component add rustfmt-preview
   - rustfmt --version
-  - |
-    if [[ $(cargo tarpaulin --version) != *0.8.4 ]]
-    then
-      cargo install --force cargo-tarpaulin
-    fi
+  - curl -fsSL https://github.com/xd009642/tarpaulin/releases/download/0.9.0/cargo-tarpaulin-0.9.0-travis.tar.gz | tar xz
+  - mv cargo-tarpaulin ~/.cargo/bin/
 
 before_script:
   - cd tests