r143478 -- gsl: update version 2.0->2.1

[petr]

The update to gsl @2.1 seems to have broken binary compatibility with dependent ports. So they need to be revision bumped. I see this for `nco` and `qgis` but there are very probably more. See below. 

~petr


{{{
[...]
--->  Installing gsl @2.1_0
--->  Cleaning gsl
--->  Removing work directory for gsl
--->  Deactivating gsl @2.0_0
--->  Cleaning gsl
--->  Removing work directory for gsl
--->  Activating gsl @2.1_0
x ./
[...]
--->  Cleaning gsl
--->  Removing work directory for gsl
--->  Updating database of binaries
--->  Scanning binaries for linking errors
Could not open /opt/local/lib/libgsl.0.dylib: Error opening or reading file (referenced from /Applications/MacPorts/QGIS.app/Contents/PlugIns/qgis/libgeorefplugin.so)
--->  Found 12 broken file(s), matching files to ports   
--->  Found 2 broken port(s), determining rebuild order
--->  Rebuilding in order
     nco @4.5.3 +accelerate
     qgis @2.12.0 +postgresql94
--->  Computing dependencies for nco.
[...]