Alpine: add tk as build dependency

[JayH5]

In #127, tcl-dev and tk-dev were added as build dependencies so that the tkinter module could be built.

Unfortunately, the Alpine Linux tk-dev package doesn't include the actual tk libraries (only the headers) so the tkinter module still fails to build. The tk package does include the relevant libraries. This is just an issue with the Alpine packages that should probably be fixed upstream (tk-dev should maybe depend on tk).

From the Travis logs for #127 for python:2.7-alpine:

Python build finished, but the necessary bits to build these modules were not found:
_bsddb             _tkinter           bsddb185        
dbm                dl                 gdbm            
imageop            nis                sunaudiodev     

From the Travis logs for #127 for python:2.7-docker:

Python build finished, but the necessary bits to build these modules were not found:
_bsddb             bsddb185           dbm             
dl                 gdbm               imageop         
sunaudiodev

From the Travis logs for this PR, after adding the tk package for python:2.7-alpine:

Python build finished, but the necessary bits to build these modules were not found:
_bsddb             bsddb185           dbm             
dl                 gdbm               imageop         
nis                sunaudiodev                        

This is a fairly obscure issue and I doubt many people (I am not one of them) are using tkinter on the Alpine images. I just had somehow had this issue with these packages before. Without this change the installation of tcl-dev and tk-dev is effectively not doing anything.

[tianon]

Aha, very nice catch! LGTM

[yosifkit]

LGTM