Bump typescript version

[michaelsauter]

Fixes #439.

Output now:

INFO: Sensor TypeScript analysis [javascript] (done) | time=7537ms
INFO: Sensor JavaXmlSensor [java]
INFO: Sensor JavaXmlSensor [java] (done) | time=1ms
INFO: Sensor SonarTS [typescript]
INFO: Since SonarTS v2.0, TypeScript analysis is performed by SonarJS analyzer v6.0 or later. No TypeScript analysis is performed by SonarTS.
INFO: Sensor SonarTS [typescript] (done) | time=0ms

Build is still green, test run seems fine.

However, there is a warning:

npm WARN [email protected] requires a peer of typescript@>= 2.3.1 < 3 but none is installed. You must install peer dependencies yourself.

See also angelozerr/tslint-language-service#77.

I would still merge this as run is fine, whereas previously we had an exception in the log output, even though the build finished successfully as well.