Upgrade to tern 0.21

Tern 0.21 was realeased on 18-02-2017

It contains some improvements (from http://ternjs.net/) : 

• Support JSDoc-style object property docs in doc_comments plugin.
• Various fixes and completions in the type definitions.
• Fix bug where server self-resets left old state on AST.
• Improve and document the webpack plugin.
• Fix issue where file offsets were interpreted wrong when the file contained astral plane characters.

Thank's for making eclipse great !