Update contributor dependencies (mlflow#2188)

* Bump sphinx-click from 2.3.0 to 2.3.1

When running the command `pip install -r dev-requirements.txt`, I
received the error that 2.3.0 had a transient dependency incompatiblity
issue. To resolve this issue, I bumped the version from 2.3.0 to 2.3.1

* Add gitpython to dev-requirements.txt

pypi-publisher pulls in an older and incompatible version of gitpython.
When following the contributors guide, it causes `python install -e .`
to fail due mlflow requiring the gitpython version being greater than 2.1.0

* Update generate-protos to use protobuf 3.6.1 also

If using something such as `brew install protobuf@3.6`,
3.6.1 is installed. Updated the `generate-protos.sh`
script to accept 3.6.1 and 3.6.0.

* Added "syntax = proto2" to databricks.proto

protoc complained that `databricks.proto` did not specifcy the
syntax. Other protobuf files such as `service.proto` do specify `syntax
= proto2`. Therefore, I've added it to `databricks.proto`

* Added success message to `test-generate-protos.sh`

* Update generate-protos.sh error message for 3.6.0 and 3.6.1

The error message does not match the conditional. We require either
3.6.0 or 3.6.1 and the message should match.

dev-requirements.txt

@@ -2,7 +2,8 @@
 sphinx==2.2.1
 sphinx_rtd_theme==0.4.3
 sphinx-autobuild==0.7.1
-sphinx-click==2.3.0
+sphinx-click==2.3.1
+gitpython>=2.1.0
 nose
 codecov
 coverage

generate-protos.sh

@@ -2,8 +2,8 @@
 
 set -ex
 PROTOC_VERSION="$(protoc --version)"
-if [ "$PROTOC_VERSION" != "libprotoc 3.6.0" ]; then
-	echo "Must have libprotoc version 3.6.0."
+if [ "$PROTOC_VERSION" != "libprotoc 3.6.0" || "$PROTOC_VERSION" != "libprotoc 3.6.1" ]; then
+	echo "Required libprotoc versions to be 3.6.0 or 3.6.1 (preferred)."
 	echo "We found: $PROTOC_VERSION"
 	exit 1
 fi

mlflow/protos/databricks.proto

@@ -1,3 +1,5 @@
+syntax = "proto2";
+
 import "google/protobuf/descriptor.proto";
 import "scalapb/scalapb.proto";