Remove the GMT6.0 dependency information from README (#1826)

Author: seisman

README.rst

@@ -63,10 +63,6 @@ publication quality maps and figures. It provides a Pythonic interface for the
 `Generic Mapping Tools (GMT) <https://github.com/GenericMappingTools/gmt>`__, a
 command-line program widely used in the Earth Sciences.
 
-We rely heavily on new features that have been implemented in GMT 6.0. In particular,
-a new *modern execution mode* that greatly simplifies figure creation. **These features
-are not available in the 5.4 version of GMT**.
-
 Project goals
 -------------