added a webjarDirectoryName parameter to make the context from which

This is a trivial change built on top of recently merged

https://github.com/searls/jasmine-maven-plugin/pull/221

Currently, all webjar scripts are served from a context named "webjar"  Spring Boot webapps require this to be "webjars".  This pull request introduces a single new parameter to JasmineConfiguration, webjarDirectoryName , to allow the context name to be configured.