Escape $

Recent changes in Julia's string interpolation has made it necessary to quote the $ character. 

Without this change, we get, using latest master, `Version 0.2.0` , commit: `5119472` 

```jlcon
julia> using Mustache
ERROR: syntax: invalid interpolation syntax: &
 in include_from_node1 at loading.jl:89
 in include_from_node1 at loading.jl:89
 in reload_path at loading.jl:109
 in require at loading.jl:48
at /Users/aviks/.julia/Mustache/src/utils.jl:48
at /Users/aviks/.julia/Mustache/src/Mustache.jl:5
```