Set prompt using double quotes

[jmitchell]

Fixes #5 in gnome-terminal.

With this change echo $PROMPT produces %(?,%{%}λ,%{%}λ) %{%}%n %{%}[%3~] %{%}→ %{%}. Otherwise, with the existing single quotes substitution is deferred:

${LAMBDA}\
 %{$fg_bold[$USERCOLOR]%}%n\
 %{$fg_no_bold[magenta]%}[%3~]\
 $(check_git_prompt_info)\
%{$reset_color%}

The substitution delay explains why several have confirmed the workaround of removing local before LAMBDA's assignment fixes it. I'm not sure why some terminals behave differently, but this fix will likely be portable and unobtrusive.

[halfo]

Hi! Thanks for the pull request. As I couldn't reproduce the issue, I asked few users here #5 if this patch solves the problem. I'll wait until they verify.

[halfo]

Thanks for the help @jmitchell :)