Ts doc versioning in github and travis release branch

[fboucquez]

Hi @rg911

This is my PR for GitHub multiple version documentation and the release branch CD. Let's assume the current version to be released is 0.17.4

TS-Docs:

Travis will push the ts-docs into the folder version, e.g. https://nemtech.github.io/symbol-sdk-typescript-javascript/0.17.4. The push only runs in master when running the node_js 8 job.

Releases from release branch:

The plan is, every time we want to release we create a PR from master to the release branch. The PR will include the changelog update explaining the new features.

Once PR is approved, Travis will pick up it, build and then it will perform a release: The release script (executed only when the branch is release and node js is version 8) will automatically:

1. git tag the to-be-released version (e.g: v0.17.4)
2. npm publish the current version (0.17.4)
3. upgrade the package.json version using npm version patch (e.g: 0.17.5)
4. Commit
5. Push commits into post-release branch (package.json will have the new 0.17.5)
6. Push the tag (v0.17.4)

Once Travis finishes, we have a few manual steps to be done:

1. Create and merge a PR from post-release to master. We need to remember to do this step. We could try pushing directly from Travis to master in step 5 but I think GitHub will complain.
2. Create a release in GitHub from the already created tag (0.17.4)
3. Send a Slack notification to sig-client

Note 1: We don't need to manually upgrade any version. Travis upgrades it.
Note 2: The release process only runs when pushing into the release branch. It doesn't run in any other branch