Update versions of dependencies

[DaniRey]

There is new versions for almost everything

SBT
0.13.15 -> 0.13.16

(Migration to SBT 1.0 might be quite an effort, because we use Plugins. Also the cross-version build
for the JSON module is not simple. Currently there is not much value in upgrading to 1.0
For an open-source project the build should mainly be in a way, that everyone can use it. Currently SBT 0.13.x is fine as build tool for an open-source project in the Scala community)

Scala

• 2.12.3 -> 2.12.7
• 2.11.11 -> 2.11.12
• 2.10.6 -> 2.10.7

ScalaTest 3.0.4 -> 3.0.5
Selenium 3.6.0 -> 3.14.0
HtmlUnit 2.27 -> 2.32.1
Play JSON 2.6.6 -> 2.6.10

Plugins
xsbt-web-plugin (Jetty for integration tests) 2.1.0 -> 4.0.2
sbt-mima-plugin (Migration Manager) 0.1.18 -> 0.3.0

Only relevant for integration tests of ScalaWebTest itself
Javax Servlet API 3.01 -> 4.0.1