fix codec enumeration (in README)

Upon further inspection of the Blosc source code the changes made to the
header README in 8707b4c15b9ee2d553937c17a3c4e78c7fd87738 must be reverted,
my bad. As it turns out, there is now one set of integers used internally by
blosc as arguments to compress and another that is actually used to identify
the codec in the header. I presume that since LZ4 and LZ4HC have the same
decompressor, the same integer (namely ``1``) is used for both. Updated the
docs to account for that.

Also, it turns out that the field that was previously used for the version of 
the blosclz is now more general and stores the version of the codec used. 
Updated the README to reflect that change.