Supporting seed

First run:
```
[13:33:10][giorgio@Bipbip:~/code/eris]$ vendor/bin/phpunit --filter ReferencePhpImplementation examples/SumTest.php
PHPUnit 4.3.5 by Sebastian Bergmann.

Configuration read from /home/giorgio/code/eris/phpunit.xml

F
Reproduce with:
ERIS_SEED=1418041993183336 vendor/bin/phpunit --filter ReferencePhpImplementation examples/SumTest.php


Time: 277 ms, Memory: 6.50Mb

There was 1 failure:

1) ListConcatenationTest::testEqualToReferencePhpImplementation
Summing 42 and 616
Failed asserting that 659 matches expected 658.

/home/giorgio/code/eris/examples/SumTest.php:55
/home/giorgio/code/eris/src/Eris/Quantifier/Evaluation.php:48
/home/giorgio/code/eris/src/Eris/Quantifier/RoundRobinShrinking.php:45
/home/giorgio/code/eris/src/Eris/Quantifier/ForAll.php:69
/home/giorgio/code/eris/src/Eris/Quantifier/Evaluation.php:50
/home/giorgio/code/eris/src/Eris/Quantifier/ForAll.php:71
/home/giorgio/code/eris/examples/SumTest.php:56

FAILURES!
Tests: 1, Assertions: 440, Failures: 1.
```
Second run:
```
[13:33:13][giorgio@Bipbip:~/code/eris]$ ERIS_SEED=1418041993183336 vendor/bin/phpunit --filter ReferencePhpImplementation examples/SumTest.php
PHPUnit 4.3.5 by Sebastian Bergmann.

Configuration read from /home/giorgio/code/eris/phpunit.xml

F
Reproduce with:
ERIS_SEED=1418041993183336 vendor/bin/phpunit --filter ReferencePhpImplementation examples/SumTest.php


Time: 260 ms, Memory: 6.50Mb

There was 1 failure:

1) ListConcatenationTest::testEqualToReferencePhpImplementation
Summing 42 and 616
Failed asserting that 659 matches expected 658.

/home/giorgio/code/eris/examples/SumTest.php:55
/home/giorgio/code/eris/src/Eris/Quantifier/Evaluation.php:48
/home/giorgio/code/eris/src/Eris/Quantifier/RoundRobinShrinking.php:45
/home/giorgio/code/eris/src/Eris/Quantifier/ForAll.php:69
/home/giorgio/code/eris/src/Eris/Quantifier/Evaluation.php:50
/home/giorgio/code/eris/src/Eris/Quantifier/ForAll.php:71
/home/giorgio/code/eris/examples/SumTest.php:56

FAILURES!
Tests: 1, Assertions: 440, Failures: 1.
```