Fix the use of immutable type for Julia 0.1.2

This solves on Julia 0.1.2
```
julia> using BioSeq
ERROR: syntax: extra token after end of expression
 in include_from_node1 at loading.jl:76
 in include_from_node1 at loading.jl:76
 in reload_path at loading.jl:96
 in require at loading.jl:48
at /home/dzea/.julia/BioSeq/src/translate.jl:215
```