code_checks.sh does not work as documentation says
#### Code Sample, a copy-pastable example if possible

```
$ ./ci/code_checks.sh lint
inside ./ci/code_checks.sh
NOT Linting. To lint use: LINT=true ./ci/code_checks.sh lint
```
#### Problem description

The current `code_checks.sh` contains this line near the top:
```
[[ $LINT ]] || { echo "NOT Linting. To lint use: LINT=true $0 $1"; exit 0; }
```
so the script automatically exits unless LINT is defined. The following are examples of calls that give expected results
```
LINT=true ./ci/code_checks.sh lint
LINT=true ./ci/code_checks.sh patterns
LINT=true ./ci/code_checks.sh doctests
LINT=true ./ci/code_checks.sh dependencies
```
but this requirement is neither intuitive nor mentioned in `contributing.rst` or the usage comments in `code_checks.sh`. Also, the travis tests are calling `./ci/code_checks.sh` with no parameters so nothing is happening there.

#### Expected Output

Output of lint

#### Output of ``pd.show_versions()``

<details>

No module named 'dask'

INSTALLED VERSIONS
------------------
commit: 99df7da9ef5b3b210f3045ee19279808368def28
python: 3.6.7.final.0
python-bits: 64
OS: Darwin
OS-release: 18.0.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.24.0.dev0+1076.g99df7da9e
pytest: 4.0.0
pip: 10.0.1
setuptools: 40.6.2
Cython: 0.29
numpy: 1.15.4
scipy: 1.1.0
pyarrow: 0.11.1
xarray: 0.11.0
IPython: 7.1.1
sphinx: 1.8.2
patsy: 0.5.0
dateutil: 2.7.5
pytz: 2018.7
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.8
feather: None
matplotlib: 3.0.1
openpyxl: 2.5.9
xlrd: 1.1.0
xlwt: 1.2.0
xlsxwriter: 1.1.2
lxml: 4.2.5
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.14
pymysql: 0.9.2
psycopg2: None
jinja2: 2.10
s3fs: 0.1.6
fastparquet: 0.1.6
pandas_gbq: None
pandas_datareader: None
gcsfs: 0.1.2
</details>
