Fix tests about vspec#pretty_string not to fail on Travis CI

[kana]

Tests about vspec#pretty_string are failed on Travis CI.

t/failure-messages-with-special-characters.t .. 1/?
not ok 1
# --- /dev/fd/62  2014-03-16 09:40:41.995094427 +0000
# +++ /dev/fd/61  2014-03-16 09:40:41.995094427 +0000
# @@ -20,6 +20,6 @@
#  #     Expected value: "foo\\\\bar"
#  not ok 6 - vspec shows strings with other special characters in a decodable style
#  # Expected "foo\x80\X7fbar" ==# 'foo\x80\X7fbar'
# -#       Actual value: "foo\x80\x7Fbar"
# +#       Actual value: "foo<80>\x7Fbar"
#  #     Expected value: "foo\\x80\\X7fbar"
#  1..6

But the tests are not failed on my machine. It might be caused by 'isprint' (which has a different default value based on the current platform) and/or 'encoding'. And I suspect both options have different values to the ones on my machine.

• Firstly, 0x80 is treated as printable on Travis CI. So that 'isprint' is configured to some value other than the default.
• Secondly, 'encoding' on Travis CI seems to be set to utf-8 or other Unicode encodings, because 0x80 is shown as <80> in the result.

[kana]

TODO: Check the result of :scriptnames, 'isprint', 'encoding' and $LANG on Travis CI. Travis CI might has a system-wide vimrc or a special $LANG.

[kana]

• $LANG ==> "en_US.UTF-8"
• &encoding ==> "utf-8"
• &isprint ==> "@,161-255"
• :scriptnames ==>
  1. ~/build/kana/vim-vspec/,,1721.vim
  2. ~/build/kana/vim-vspec/autoload/vspec.vim
  3. /tmp/vqOIzZu/0

[kana]

Fixed.