make sure that we output decimal for duration

make sure that we output decimal for duration

Fix mocktracer exception handling. (opentracing#118)

Preparing for release 1.5.1.

Preparing for release 1.5.0. (opentracing#94)

Bump patch version.

Bump version.

Prepare for release. (opentracing#77)

Make MockTracer's behavior more customizable. (opentracing#59)

Set up CI testing. (opentracing#44)

* Set up CI testing.

* Add osx to build matrix.

* Switch to run on xcode9.