Planemo + R = 'ascii' codec can't encode character u'\u2018'

Hi all,

I'm used to wrap some R script/libary. And R is used to display in the stderr this kind of message:

```
Warning messages:
1: replacing previous import by ‘ggplot2::unit’ when loading ‘Hmisc’
2: replacing previous import by ‘ggplot2::arrow’ when loading ‘Hmisc’
3: replacing previous import by ‘scales::alpha’ when loading ‘Hmisc’
```

or

```
Error in library("foobar") : there is no package called ‘foobar’
```

`planemo test` seems to dislike those `‘` and `’` when displaying the logs both in the stacks and in the HTML report:

```
======================================================================
ERROR: IPO for xcmsSet ( ipo4xcmsSet ) > Test-1
----------------------------------------------------------------------
Traceback (most recent call last):
File "/tmp/tmpxRw9ls/galaxy-dev/test/functional/test_toolbox.py", line 299, in test_tool
self.do_it( td )
File "/tmp/tmpxRw9ls/galaxy-dev/test/functional/test_toolbox.py", line 78, in do_it
raise e
UnicodeEncodeError: 'ascii' codec can't encode character u'\u2018' in position 121: ordinal not in range(128)
```

Consequence: the test fails even if the script is processed normally and you don't know exactly what append.

For the record:
I managed to go through the first case using `suppressWarnings()`:
`suppressWarnings( suppressPackageStartupMessages( stopifnot( library(pkg, quietly=TRUE, logical.return=TRUE, character.only=TRUE))))`

That was my feedback!
Cheers