ARROW-3551: Update MapD to OmniSci on Powered By page

Author: Todd Mostak <tmostak@gmail.com>

Closes #2787 from tmostak/patch-1 and squashes the following commits:

79cdd6c12 <Todd Mostak> Update MapD to OmniSci on Powered By page
diff --git a/powered_by.md b/powered_by.md
index 1e9747c..cdfe93a 100644
--- a/powered_by.md
+++ b/powered_by.md
@@ -85,9 +85,9 @@
   support for structured data. Uses the Arrow IPC format and targets the Arrow
   memory layout in its analytic functions. This work is part of the [GPU Open
   Analytics Initiative][11]
-* **[MapD][10]:** in-memory columnar SQL engine designed to run on GPUs. MapD
-  supports Arrow for data ingest and data interchange via CUDA IPC
-  handles. This work is part of the [GPU Open Analytics Initiative][11]
+* **[OmniSci][10] (formerly MapD):** In-memory columnar SQL engine designed to run
+  on both GPUs and CPUs. OmniSci supports Arrow for data ingest and data interchange
+  via CUDA IPC handles. This work is part of the [GPU Open Analytics Initiative][11]
 * **[pandas][12]:** data analysis toolkit for Python programmers. pandas
   supports reading and writing Parquet files using pyarrow. Several pandas
   core developers are also contributors to Apache Arrow.
@@ -140,7 +140,7 @@
 [7]: https://spark.apache.org/
 [8]: https://github.com/locationtech/geomesa
 [9]: https://www.dremio.com/
-[10]: https://github.com/mapd/mapd-core
+[10]: https://github.com/omnisci/mapd-core
 [11]: https://gpuopenanalytics.com/
 [12]: https://pandas.pydata.org
 [13]: https://quiltdata.com/
@@ -158,4 +158,4 @@
 [25]: https://github.com/uwdata/falcon
 [26]: https://fastdata.io/
 [27]: https://github.com/apache/arrow/edit/master/site/powered_by.md
-[28]: https://github.com/uber/petastorm
\ No newline at end of file
+[28]: https://github.com/uber/petastorm