fix minimal Nephia version

This module requires Nephia 0.32 or higher. But I failed test with Nephia 0.35.


```
[0.35]
% ./Build test
t/00_compile.t ..... ok
t/01_session.t ..... # PSGI error: Can't call method "env" on an undefined value at /home/syohei/.cpanm/work/1374457273.23210/Nephia-Plugin-PlackSession-0.01/blib/lib/Nephia/Plugin/PlackSession.pm line 12.
t/01_session.t ..... 1/?
#   Failed test 'GET /'
#   at t/01_session.t line 28.
# 500
# undef

#   Failed test at t/01_session.t line 29.
#          got: 'Can't call method "env" on an undefined value at /home/syohei/.cpanm/work/1374457273.23210/Nephia-Plugin-PlackSession-0.01/blib/lib/Nephia/Plugin/PlackSession.pm line 12.
# '
#     expected: '1'
# PSGI error: Can't call method "env" on an undefined value at /home/syohei/.cpanm/work/1374457273.23210/Nephia-Plugin-PlackSession-0.01/blib/lib/Nephia/Plugin/PlackSession.pm line 12.

#   Failed test 'GET /'
#   at t/01_session.t line 30.
# 500
# undef

#   Failed test at t/01_session.t line 31.
#          got: 'Can't call method "env" on an undefined value at /home/syohei/.cpanm/work/1374457273.23210/Nephia-Plugin-PlackSession-0.01/blib/lib/Nephia/Plugin/PlackSession.pm line 12.
# '
#     expected: '2'
# Looks like you failed 4 tests of 4.
t/01_session.t ..... Dubious, test returned 4 (wstat 1024, 0x400)
Failed 4/4 subtests
t/02_flavor.t ...... ok
t/03_flavor_dbi.t .. ok

Test Summary Report
-------------------
t/01_session.t   (Wstat: 1024 Tests: 4 Failed: 4)
  Failed tests:  1-4
  Non-zero exit status: 4
Files=4, Tests=11,  1 wallclock secs ( 0.03 usr  0.02 sys +  0.71 cusr  0.09 csys =  0.85 CPU)
Result: FAIL
Failed 1/4 test programs. 4/11 subtests failed.
```

I success tests with Nephia 0.36.
```
[0.36]
% ./Build test
t/00_compile.t ..... ok
t/01_session.t ..... ok
t/02_flavor.t ...... ok
t/03_flavor_dbi.t .. ok
All tests successful.
Files=4, Tests=11,  2 wallclock secs ( 0.10 usr  0.02 sys +  1.02 cusr  0.16 csys =  1.30 CPU)
Result: PASS
```