fix mimic joint limits

[onionsflying]

Hi,
I'm using gazebo to simulate robotiq_2f_140_gripper with the MimicJointPlugin. Some problems occurred. Then I found that the mimic joint limits were wrong. For example, the limits of left_inner_knuckle_joint and right_inner_knuckle_joint should be [-0.8757, 0] and [0, 0.8757], but they were both set as [0, 0.8757]. Since they are mimic joints, I think the simplest solution is to set the joint limits symmetrically.

[jproberge]

Good timing, this is exactly the same concern as #119 which was merged recently, but for another branch. Thank you, I'm merging this as this was indeed a needed fix.