Explicitly set coding for subprocess when running on macs

[xor-xor]

Fixes #36

An example of output from the process buffer that I'm getting after this change:

          8,196   0%    0.00kB/s    0:00:00  
          8,196   0%    0.00kB/s    0:00:00 (xfr#1, to-chk=100/102)
          8,196   0%    0.00kB/s    0:00:00 (xfr#1, to-chk=95/102) 
          8,196   0%    0.00kB/s    0:00:00 (xfr#2, to-chk=99/102)
    254,812,164   5%  243.00MB/s    0:00:18  
    491,659,268   9%  234.55MB/s    0:00:18  
    741,744,644  14%  235.87MB/s    0:00:17  
    980,557,828  19%  233.90MB/s    0:00:16  
  1,211,375,620  24%  228.18MB/s    0:00:16  
  1,446,912,004  29%  227.86MB/s    0:00:15  
  1,677,598,724  33%  219.02MB/s    0:00:14  
  1,914,839,044  38%  218.65MB/s    0:00:13  
  2,021,051,115  40%  226.30MB/s    0:00:08 (xfr#3, to-chk=98/102)
  2,021,057,263  40%  226.30MB/s    0:00:08 (xfr#4, to-chk=94/102)
  2,106,122,991  42%  209.40MB/s    0:00:13  
  2,295,522,031  46%  198.60MB/s    0:00:13  
  2,469,847,791  49%  189.03MB/s    0:00:12  
  2,640,896,751  53%  173.19MB/s    0:00:13  
  2,818,761,455  56%  169.99MB/s    0:00:12  

  [... snip ...]