Fix deprecations, update for DataFrames 0.11

[randyzwitch]

• Switched isna for ismissing,
• removed references to ASCIIString in docstrings
• Updated REQUIRES, as other CI tests suggest tests will pass using DataFrames 0.11

[randyzwitch]

@aviks I think this PR is ready to merge. Travis passes all of the tests; AppVeyor fails, but it's not due to the code I don't believe:

INFO: Testing Taro
Loaded C:\Program Files\Java\jdk1.7.0\jre\bin\server\jvm.dll
INFO: adding t1 sheet ...
INFO: adding t2 sheet ...
INFO: wrote all dataframes to C:\Users\appveyor\.julia\v0.6\Taro\test\df-test-writexl.xlsx.
ERROR: LoadError: unlink: resource busy or locked (EBUSY)
Stacktrace:
 [1] uv_error at .\libuv.jl:68 [inlined]
 [2] unlink(::String) at .\file.jl:500
 [3] #rm#7(::Bool, ::Bool, ::Function, ::String) at .\file.jl:145
 [4] (::Base.Filesystem.#kw##rm)(::Array{Any,1}, ::Base.Filesystem.#rm, ::String) at .\<missing>:0
 [5] include_from_node1(::String) at .\loading.jl:576
 [6] include(::String) at .\sysimg.jl:14
 [7] process_options(::Base.JLOptions) at .\client.jl:305
 [8] _start() at .\client.jl:371
while loading C:\Users\appveyor\.julia\v0.6\Taro\test\runtests.jl, in expression starting on line 35
================================[ ERROR: Taro ]=================================
failed process: Process(`'C:\projects\julia\bin\julia.exe' -Cx86-64 '-JC:\projects\julia\lib\julia\sys.dll' --compile=yes --depwarn=yes --check-bounds=yes --code-coverage=none --color=no --compilecache=yes 'C:\Users\appveyor\.julia\v0.6\Taro\test\runtests.jl'`, ProcessExited(1)) [1]

I don't have a Windows computer to test out a solution, but maybe that error is okay anyway? Your repo, your call.

[randyzwitch]

Sorry for all the noise, not ready for merging. While the tests pass, using Taro right now I'm already seeing issues. I'll work through the library and see if I can fix these.

[aviks]

Thanks for tackling this. That dataframe conversion code isn't the best...

[randyzwitch]

You can merge this now @aviks, it was user error that was making my code not run correctly. Taro.jl reads blank cells in correctly as missing.

I'm going to try and keep working on this library, but I also don't want to hold up getting a DataFrames 0.11 compliant Excel reader out there (currently, all are broken for julia 0.0/DataFrames 0.11)