Update to 0.6.6

changelog:
0.6.6
  * Insert braces around macro expansions to prevent breakage (#7).
  * Support \operatorname and \DeclareMathOperator (rekka) (#17).
  * Support \providecommand (#15).
  * Fixed spacing bugs in pandoc rendering (#24).
  * Ignore \hline at end of array row instead of failing (#19).
( #24 => https://github.com/jgm/texmath/issues/24 )
( #19 => https://github.com/jgm/texmath/issues/19 )
( #17 => https://github.com/jgm/texmath/pull/17 )
( #15 => https://github.com/jgm/texmath/issues/15 )
( #7 => https://github.com/jgm/texmath/issues/7 )