Find pcl_conversions if building against PCL 1.7.0+

It gets included depending on the PCL version in src/ros/pcl_bridge.cpp
Always find pcl_conversions

Should be okay since it's a package dependency.
Use the new header with PCL 1.7.0+
Merge branch 'master' into patch-1
Merge pull request #35 from pgorczak/patch-1

Find pcl_conversions if building against PCL 1.7.0+