libcurl-devel is required for install

Looks like `pip install clodius` tried to install pyBigWig but it couldn't finish without the libcurl dev package. I installed libcurl3-dev and it works, though it looks like it's a known issue on pyBigWig (https://github.com/deeptools/pyBigWig/issues/9)

```
Collecting clodius
Downloading clodius-0.7.4.tar.gz (72kB)
100% |████████████████████████████████| 81kB 3.5MB/s
Complete output from command python setup.py egg_info:
Either libcurl isn't installed, it didn't come with curl-config, or curl-config isn't in your $PATH. pyBigWig will be installed without support for remote files.
/usr/lib/python2.7/distutils/dist.py:267: UserWarning: Unknown distribution option: 'classifier'
warnings.warn(msg)
('packages:', ['clodius', 'clodius.cli'])
pyBigWig.c: In function ‘canAppend’:
pyBigWig.c:911:11: warning: unused variable ‘foo’ [-Wunused-variable]
void *foo;
^~~
pyBigWig.c: In function ‘PyAddIntervals’:
pyBigWig.c:1004:11: warning: unused variable ‘foo’ [-Wunused-variable]
void *foo;
^~~
pyBigWig.c:998:15: warning: unused variable ‘tmp’ [-Wunused-variable]
PyObject *tmp;
^~~
/usr/bin/ld: cannot find -lcurl
collect2: error: ld returned 1 exit status
/usr/bin/ld: cannot find -lcurl
collect2: error: ld returned 1 exit status
```