Remove use of Gradle JavaFX plugin

As documented at [1], version 8.1.1 of the Gradle JavaFX plugin has
disappeared from bintray. This causes Travis CI build failures. Version
8.1.0 is still in place [2], but downgrading to it causes further build
errors.

This commit removes use of the plugin entirely in order to fix the CI
problem immediately. In the meantime, the Bitsquare UI must be launched
directly from within IDEA.

Resolves #221
See #66

[1]: https://bitbucket.org/shemnon/javafx-gradle/issue/46/received-status-code-401-from-server
[2]: http://dl.bintray.com/content/shemnon/javafx-gradle/org/bitbucket/shemnon/javafxplugin/gradle-javafx-plugin/