exporter/jaeger: thrift checksum mismatch on build with go modules

[jbub]

Describe the bug

Project that imports go.opencensus.io/exporter/jaeger and is using Go modules fails on build with:

verifying git.apache.org/thrift.git@v0.12.0: checksum mismatch
--
206 | downloaded: h1:CMxsZlAmxKs+VAZMlDDL0wXciMblJcutQbEe3A9CYUM=
207 | go.sum: h1:692K1/SsOcQvkvMRMdt60FCq2AvKpuQNM6sIeH3mN4s=

To Reproduce
Steps to reproduce the behavior:

1. Clone this repo https://github.com/jbub/censusthrift.
2. Run docker build or check the results of a failed build here https://cloud.drone.io/jbub/censusthrift/1/1/2.

Expected behavior
Expected the Go project to be built successfully.

Additional context
Original issue #993.

[jbub]

I thought i did init modules with Go 1.12 but apparently i did not ? Anyway, this commit fixed it jbub/censusthrift@d4bcd45.