Update nucleotide_count_test.py

Removed test_dna_has_no_uracil for a couple of reasons
* The module is named dna.py and in the README.md we see that DNA does not contain uracil only RNA does
* If 'U' is added to the dictionary it causes test_counts_all_nucleotides, test_empty_dna_string_has_no_nucleotides, and test_repetitive_sequence_has_only_guanosine to fail.
* I believe that counting for 'U' should also throw a ValueError

As of now the only solution I could think of was to add the following:
if nucleotype is 'U': return 0 
which seems inelegant to me.

Hope this helps,

Oniwa