Add a build for Go 1.10 and make it the default

While doing this, double-quote all version numbers so that they are
parsed as strings, not floating point numners, which is a requirement
for getting 1.10 to be recognized properly.

.travis.yml

@@ -23,8 +23,9 @@ language:
   - java  # For Bazel.
 
 go:
-  - 1.8
-  - 1.9
+  - "1.8"
+  - "1.9"
+  - "1.10"
 go_import_path: github.com/bazelbuild/sandboxfs
 
 env:
@@ -38,22 +39,30 @@ script: ./admin/travis-build.sh
 
 matrix:
   exclude:
-    # For Go 1.8, we are only interested in testing that our code builds
+    # For Go 1.8 and 1.9, we are only interested in testing that our code builds
     # correctly under that version of the language.  A single test is sufficient
     # for this, so remove all other configurations.
     - env: DO=gotools
-      go: 1.8
-    - go: 1.8
+      go: "1.8"
+    - env: DO=gotools
+      go: "1.9"
+    - go: "1.8"
+      os: osx
+    - go: "1.9"
       os: osx
 
     # For code linting, we just need to run this under a single configuration.
     # Trim all but one, and prefer Linux over macOS because it's much faster.
     - env: DO=lint
-      go: 1.8
+      go: "1.8"
+    - env: DO=lint
+      go: "1.9"
     - env: DO=lint
       os: osx
 
     # For testing the Rust implementation, we only need to build the tests using
     # one Go version.
     - env: DO=rust
-      go: 1.8
+      go: "1.8"
+    - env: DO=rust
+      go: "1.9"