Travis Reconfiguration to fix tests

[lumip]

Recently tests where failing for python 3.4 environment on travis.
We where using miniconda on travis to install gmpy2 more easily. conda install gmpy2 caused conda to update python to version 3.7 on this environment which in turn caused some test expecting a SyntaxError to fail because that SyntaxError wasn't raised anymore (my guess is that until recently either conda did only update the python version to 3.6).
Trying to fix this surfaced two more issues:

• dependencies in requirements.txt where not available in python 3.4
• a lot of SyntaxErrors in regular parts of code with python 3.4

I made the following changes:

• Dropped official support for 3.4 in setup.py as our code is not compatible
• Fixed problems tests had with python 3.7 and added travis test configuration
• Completely removed usage of conda during tests on travis. For tests we need to be able to configure exactly what environment the tests are executed in. If miniconda changes this in the background, it's not useful.
• No longer installing requirements.txt in travis tests. The travis.yml is configured so that different configurations of extras (as defined in setup.py) are tested (e.g., native types vs gmpy2 for faster/more precise arithmetics). Installing all requirements from requirements.txt beforehand defeats the purpose of this. (this does, however, open us up to the issue of dependencies becoming incompatible in future updates, for which I don't have a nice solution yet)

[coveralls]

Pull Request Test Coverage Report for Build 1587

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 93.549%

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Files with Coverage Reduction	New Missed Lines	%
qupulse/utils/types.py	3	100.0%

Totals
Change from base Build 1561:	0.0%
Covered Lines:	4616
Relevant Lines:	4869

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💛 - Coveralls

[terrorfisch]

Looks good to me.