Use <<= instead of deprecated << in SPARQL parser

Allows to use newer pyparsing (>=2.0.1) without getting deprecation warnings:

```
pyparsing.py:3546: DeprecationWarning: Operator '<<' is deprecated, use '<<=' instead
  ret << Group( Suppress(opener) + ZeroOrMore( ignoreExpr | ret | content ) + Suppress(closer) )
```

Addresses inveniosoftware/invenio#2077