dont memoaize the namespace on the class level

This does not affect much the performance. follow benchmarks

before PR
```ruby
/opt/boxen/rbenv/versions/1.9.3-p385-perf/bin/ruby ./performance/cpu.rb
Rehearsal ----------------------------------------------------
FindRunner:        0.860000   0.090000   0.950000 (  1.224831)
FetchMissRunner:  12.160000   0.760000  12.920000 ( 83.156324)
FetchHitRunner:   13.250000   0.850000  14.100000 ( 84.238404)
------------------------------------------ total: 27.970000sec

                       user     system      total        real
FindRunner:        0.840000   0.080000   0.920000 (  1.190167)
FetchMissRunner:  12.010000   0.730000  12.740000 ( 82.843079)
FetchHitRunner:   13.250000   0.840000  14.090000 ( 84.364599)
```

after PR
```ruby
/opt/boxen/rbenv/versions/1.9.3-p385-perf/bin/ruby ./performance/cpu.rb
Rehearsal ----------------------------------------------------
FindRunner:        0.890000   0.090000   0.980000 (  1.258933)
FetchMissRunner:  12.580000   0.780000  13.360000 ( 84.006055)
FetchHitRunner:   13.760000   0.890000  14.650000 ( 85.127375)
------------------------------------------ total: 28.990000sec

                       user     system      total        real
FindRunner:        0.860000   0.080000   0.940000 (  1.214727)
FetchMissRunner:  12.270000   0.750000  13.020000 ( 82.983523)
FetchHitRunner:   13.810000   0.880000  14.690000 ( 85.356588)
```

cc @camilo @hornairs @dylanahsmith