Fix PNG support on Travis

[gutomaia]

Travis is breaking tests 'cause of the lack of PIL PNG on PIP.

Witch could be called the 3P problem (3Ps with 3 letters each, that is just Pain Phuck Parody).

Anyway. Just someone that could fix this:

91 --------------------------------------------------------------------
92 PIL 1.1.7 SETUP SUMMARY
93 --------------------------------------------------------------------
94 version 1.1.7
95 platform linux2 2.7.3 (default, Aug 1 2012, 05:16:07)
96 [GCC 4.6.3]
97 --------------------------------------------------------------------
98 *** TKINTER support not available
99 --- JPEG support available
100 *** ZLIB (PNG/ZIP) support not available
101 *** FREETYPE2 support not available
102 *** LITTLECMS support not available
103 --------------------------------------------------------------------
104

[klaussilveira]

Fixed on bd3912b.